1. Field of the Invention
The present invention relates to a process for producing at a low cost polyvinyl alcohol (PVA) fiber having high strength and to a novel fiber obtained by said process.
2. Description of the Background
PVA fibers have higher strength and higher elastic modulus than polyamide, polyester and polyacrylonitrile fibers and have been principally used in various industrial fields. In recent years PVA has also been used as a replacement for asbestos in reinforcing cement and for reinforcing rubber, plastics, and like materials.
Several processes are known for obtaining PVA fibers having still higher strength. For example U.S. Pat. No. 4,440,711 discloses a process which employs the concept of gel-spinning and super-drawing, which was first used with diluted solutions of high-molecular-weight polyethylene. U.S. Pat. No. 4,698,194 shows a process of using an organic solvent for a dope solution and conducting dry-jet-wet spinning of the dope.
The dry-jet-wet spinning employed in the above inventions comprises extruding PVA solution through a nozzle and, via an air layer, introducing the extruded solution into a coagulating bath. Although the process utilizes a diluted solution, the solution cannot be a very diluted one, since the extruded solution must pass through an air layer while maintaining the form of continuous streams.
With a very diluted PVA solution, the polymer solution tends to stick to the spinneret surfaces and stable spinning is difficult to achieve. Where a spinneret having holes with small pitch is used, the polymer streams, just after the extrusion, tend to contact each other and stick to each other in the air and stable spinning is not possible. If spinning is ever conducted, the solidified filaments obtained become what are called stuck filaments, which will undergo filament breakage upon heat drawing at high ratios, which makes it difficult to obtain high-strength fiber. To solve this problem, the spinneret used must have a hole allocation with a large pitch, which means the spinneret cannot have very many holes, thus creating another problem of high production costs in commercial production. A large-diameter spinneret may be employed in order to increase the number of holes per spinneret, but it presents disadvantages of difficult handling and, in particular, extrusion tends to be uneven. Accordingly, the dry-jet-wet spinning is associated with several problems from the viewpoint of commercial production.
Wet spinning of PVA fiber is also known. See, for example Japanese Patent Publication No. 16675/1968. In wet spinning, although it creates no such problems as associated with dry-jet-wet spinning, no fiber has yet been obtained which has high strength and high modulus. A study has now been conducted as to why conventional wet spinning cannot give a high-strength fiber. In wet spinning, the spinneret used and the dope piping connected thereto are immersed in a coagulating bath, and hence the coagulating bath temperature and the dope temperature influence each other. With a large difference between the two temperatures, an unevenness of temperature exists between the dope piping and the center and peripheral part of the spinneret used. This temperature difference results in a viscosity unevenness in the dope, which makes regular and uniform extrusion impossible. This fact is one of the causes which obstructs the production of high-strength PVA fiber, and is very much so in particular with a low coagulating bath temperature of 20.degree. C. or below. A need therefore continues to exist for an improved method of preparing PVA fibers.